Optical fiber technology has been a major driver behind the information technology revolution and the rapid progress in global telecommunications. Fiber optics have become a standard transmission medium for voice, video and data signals, serving as a foundation for virtually every form of communication network, from interoffice to trans-oceanic.
Three general trends in signal processing have recently emerged in communication and sensing applications. The first is recognized in increased signal bandwidths that are no longer compatible with conventional electronics. The second trend is driven by large dynamic range expected from a mixed (analog/digital) signal processor. Finally, the standard (incoherent) processing is being eliminated with the introduction of fully-coherent optical links.
Parametric mixers are recognized as a core processing technology in this regard and have been investigated in silica, silicon and semiconductor platforms. One example of a parametric mixer is the fiber optic parametric amplifier (FOPA), can be used in lightwave systems for applications such as optical amplification, phase conjugation, and wavelength conversion. In principle, FOPAs can provide uniformly high gain over a wide range of wavelenths (>100 nm). Further, FOPAs add little noise to the amplified signal, with a noise figure as low as 0 dB when operated in the phase-sensitive mode and 3 dB in the phase-insensitive mode. However, in practice, these advantages are adversely impacted by factors that include irregularities in fiber construction and performance.
With octave-wide bandwidth and high figure of merit (FoM) driven by a long interaction length, fiber-optic mixers represent particularly important class of processing engines. They allow for power-efficient access to ultrafast signals, with applications ranging from Tb/s channel transport, propagation impairment reversal, packet manipulation, and, more recently, complex analog signal acquisition and wavelength conversion to non-conventional bands. Highly-nonlinear fiber (HNLF) has been the most important mixer platform to date, and is responsible for a majority of the advances made in low-power, high-bandwidth parametric devices.
Regardless of specific device functionality, its power efficiency, noise and bandwidth metrics are uniquely determined by the phase-matching condition of the underlying four-photon mixing (FPM) processes. Phase matching in fiber parametric mixers is defined by the combination of material and waveguide dispersion characteristics. The latter is recognized as a powerful tool in tailoring the mixer response over large spectral range: a conventional HNLF index profile allows for significant dispersion change even with small transverse fiber geometry alterations. While this sensitivity is clearly desirable for phase-matched design, it also introduces the fundamental phase-matching limit. In addition to deterministic core variation, HNLF fabrication is accompanied by inherently stochastic microscopic fluctuation that leads to considerable dispersion variation. As a result, phase-matching variance along the fiber length severely impacts the performance of a mixer relying on long interaction length, denying the advantage of fiber-based mixer construction. Specifically, distant-band parametric mixer relying on negative fourth-order dispersion is recognized as the most sensitive to such dispersive fluctuations. For this reason, recent demonstrations have exclusively relied on standard dispersion-shifted fiber (DSF), rather than existing HNLF type. To maintain the FoM of a DSF mixer, pump powers had to be scaled by nearly an order of magnitude, as required by the ratio between HNLF and DSF nonlinear coefficients.
Recognizing this limitation, various post-fabrication correction schemes have been demonstrated. Such approaches typically require longitudinal mapping of local dispersion fluctuations and must be followed by either selection/concatenation of useful sections or localized dispersion equalization. In contrast, pre-fabrication measures for HNLF design with inherent resilience to local geometry fluctuation are highly desirable yet remain largely unexplored. Rare attempts to rectify the dispersion fluctuation via unconstrained optimization were driven by purely mathematical formulation and have resulted in index profiles that are either not manufacturable using commercially practical methods or possess nonlinear coefficients similar to that of the standard fibers.
In order to realize the true potential of parametric mixers, including FOPAs, the need exists for an optical fiber that is compatible with practical HNLF manufacturing techniques that is capable of satisfying the phase matching requirements of parametric mixer devices